Download PDF file: Root and Stem and Crown Rot Oomycetes

Strengths and weaknesses
of our current knowledge
of root, stem and crown rot
oomycetes
Tim Pettitt
NPARU
University of Worcester
Strengths and weaknesses of our current knowledge of root, stem and crown rot oomycetes
• What are oomycetes?
• Oomycete root and stem diseases in horticulture
• Life‐cycles & Ecology
– Inoculum: spore types & behaviour
– Dispersal, Survival & Infection
• Disease Risks ‐ predisposition, disease thresholds & inoculum potential
• Diagnostics
• Management & Control
What are oomycetes?
• Large group of ‘Fungus‐like micro‐organisms’ – resemblance is superficial
• Representatives in virtually all terrestrial, marine and freshwater habitats
• Many spp. are saprophytes but significant proportion are pathogens of wide range of plants and animals including humans
• DNA studies show they are quite distant from true fungi ‐ more closely related to golden algae (Chrysophyceae) and diatoms
Epiphysis
(Chrysophyceae)
Oomycete – Phytophthora
Oospore
mycelium
What are oomycetes?
•
Oomycetes are NOT fungi
•
There are a number of fundamental biochemical differences ‐ probably the most important is that oomycete cell walls contain cellulose and β‐ glucans whilst fungal walls are chitin
•
Oomycete hyphae are non‐septate – lacking cross walls most fungi have abundant septa – this helps with recognition under microscope
•
The majority of oomycetes produce a very special motile swimming spore – the zoospore –
which has important characteristics shared with closely related groups but NOT the fungi
Zoospores
(Biflagellate, heterokont)
A – oomycete hypha
B – fungal hypha
www.kauridieback.co.nz/media/33561/zoospore.png Strengths and weaknesses
Research on understanding the origins etc. of oomycetes has been strong.
• Much excellent work carried out on phylogeny (understanding their place on the tree of life) and molecular diagnostics over last 15 years
• Many new species have been discovered and a good basis for understanding their complex interrelationships
• Most research has focussed (justifiably) on the ‘threats’ (novel pathogens –
implication = ‘all germs are bad’). Not enough study of ecosystem services and what keeps endemic species ‘in balance’ Why is this important?
• Effective management strategies reliant on understanding pathogen biology/ecology – there are big differences between fungi and oomycetes
• Many chemicals that give control of fungi do not work on oomycetes and vice versa
• Accurate and timely diagnosis is vital
Main oomycete root and stem rot pathogen genera are
Pythium, Phytophthora & Aphanomyces
• At least 90 species recorded worldwide on UK horticultural crop species
• Of these 28 are serious in UK and a further 26 pose a significant threat
Generalised oomycete life cycle
Variations on the theme
Chlamydospores
Hyphal swellings
Mycelium
Inoculum:
Spore types and survival structures
Cysts
Zoospores
Sporangia
Oospores
Zoospores:
•
•
•
•
Motile and free‐swimming
Can swarm together (auto aggregation)
Naturally swim upwards
Are attracted by chemicals and electric fields of host root systems
A) Aphanomyces cochlioides zoospores Chemotactically Attracted to Sugar Beet Root Tip.
(B) Aphanomyces cochlioides zoospores Aggregated on Root Tip (arrow). (Tofazzal and Taharat, 2001).
Garrison (2008) http://www.cals.ncsu.edu/course/pp728/Aphanomycescochlioides/Aphanomycescochlioides.html
Phytophthora attracted to root extract in a capillary tube next to a tube containing water (Ronaldo Dalio https://www.youtube.com/watch?v=F4slTLkhwuY)
Infection: zoospore cysts
• Often aggregate – especially on roots
• Align themselves – germinate directly towards host Not all infection is by zoospore cysts‐
•
•
some pathogen spp. do not produce zoospores
Hyphal‐tip infection not so readily measured (maybe not as photogenic!)
Pythium aphanidermatum cysts infecting a root tip
Deacon ‐ http://archive.bio.ed.ac.uk/jdeacon/microbes/zoospore.htm
Pythium violae cavity spot lesion on carrot (no zoospores)
Germinating oopspore
Phytophthora cinnamomi cysts on onion root
Hardham (2001) Aust. Plant Path 30:91‐98
Polyplanetism or repetitional‐diplanetism
A short‐term survival mechanism in adverse conditions that permits zoospore movement in ‘stages’
Direct germination
Infection
Zoospore release (Oregon State University)
Encystment ‐occurs either when zoospores reach their target host or as a reaction to adverse conditions At host surface (e.g. roots) or presence of plentiful nutrients
Cysts – form either as a precursor to infection OR as survival structures
Polyplanetic cycle
appresorium
Cysts have walls and are more resistant than zoospores and can survive in water for up to 3 months
Empty cyst cases
Motile zoospores
Indirect germination by zoospore re‐emergence
Zoospore cyst formation and cyst survival on exposure to different concentrations of peroxy‐acetic acid
PAA concentration
(mg/l)
Water source
Cyst forma on & (% viable)†
Efford
Nursery A
Nursery B
0.2
32 (82)
100 (98)
85 (98)
2.0
54 (5)
100 (30)
95 (15)
20.0
96* (0)
100* (0)
100* (0)
* Cysts malformed with ‘wrinkled’ walls
† Viability determina ons on ¼ strength PDA and are probably under‐estimates.
Dispersal
• Unlike S.O.D. and blight, the majority of root and stem rot oomycetes are not airborne (although they are still spread by wind‐driven rain!)
• Spread depends on – Scattering infested soil
– Scattering infected plant fragments & debris
– Dust & dirt containing above
– Contaminated water
Zoospores – dispersed in water
• Surface films & water‐logging
• Runoff & irrigation water & PUDDLES
Oospores/Chlamydospores/Mycelium/ Swellings & stromata
• Released from decaying plant matter and soil OM
• Can adhere to benches, floors, equipment, boots & tyres, trays/containers, Danish trolleys
Survival
Structure
Estimated survival/longevity
In soil
In water
Dry
Mycelium
Hours‐Days
Hours‐
Days
Zoospores
Hours‐Days
Hours‐
Days
‐
‐
Zoospore cysts
Days‐weeks
>3 months
‐
Chlamydospores
Years
?
+
Oospores
Years
?
+
Days‐months
?
?
Hyphal swellings
Strengths and weaknesses
This is generally a well‐established area, although there have been some improvements in our understanding of zoospore behaviour.
• Considerable bank of knowledge built up on release, survival, taxis, attachment & germination of zoospores • For many oomycetes, zoospores are the most important dispersal/infection spores but there is a bit of a ‘zoospore fixation’, and not enough work on survival structures
• Nevertheless, there is still much to learn about zoospores – what induces ‘survival encystment’? Can auto‐
aggregation and taxis be exploited for control?
Why is this important?
• Understanding the subtleties of pathogen life‐cycles identifies potential for effective disease management and avoidance
• It is important to remember that oomycetes have a range of propagules and survival strategies –
not just zoospores (important to consider the entire offensive team not just the strikers)
• Control measures aimed solely at zoospores could be ineffective against other propagules Quantifying soil inoculum
– Disease risk thresholds?
Seasonality of inoculum production:
are there optimum times to test?
160
Mean number of
Fusarium sp. cfu litre-1
140
120
Pooled results from 15 years’ irrigation water tests throughout UK
100
80
60
40
20
160
0
140
Mean numbers of oomycete cfu litre-1
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
120
2500
100
2000
Mean total fungus cfu litre-1
80
1500
60
1000
40
500
20
0
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Phytophthora inoculum production in a recirculating outdoors HNS bed
Short‐lived ‘spikes’ of inoculum = discrete high risk periods
‐ infrequent sampling could miss these
Phytophthora propagules (cfu/l)
250
200
150
100
50
0
0
50
100
150
200
250
300
Time (days)
• Going by symptoms alone not sufficient. It can take 3 months to a year for shoot symptoms to appear from a single infection event
• Testing pot effluents can reveal early infections
Inoculum density & infection
Pathogen
Host
% infection
Inoculum concentration
Ref.
Pythium aphanidermatum
Tomato
50
250
Zoospores/ml
Mitchell, 1978
Phytophthora cryptogea
Watercress
50
276
Zoospores/ml
Mitchell, 1978
Phytopythium ostracodes
(was Pythium)
Cotton
50
281
Zoospores/ml
Mitchell, 1978
Phytophthora ramorum
Range of tree hosts
50
100‐250
Sporangia/ml
Tooley et al.,
2013
Zoospores/ml
Pettitt et al.,
2001
Zoospores/ml
Pettitt 1989
Phytophthoa cryptogea
Phytophthora cactorum
Tomato
36‐750
with wounding
50
400
Strawberry
Zero!
10 000
(var. Tamella)
50
1000
(Hydroponic)
Cold‐stored 100
ditto
25
Crop susceptibility affected by growth stage, season and environment
Periods of high susceptibility likely to coincide with ‘inoculum spikes’ ‐ little data about this
Phytophthora cactorum strawberry crown rot
• Cold‐stored runners = ultra suscetible
• Traditionally a ‘spring wilt’
Pythium sylvaticum xanths root rot
•
A disease problem that used to plague late winter and early spring crops
Pythium damping‐off
• Seedlings and very young high nutrient plants susceptible
Strengths and weaknesses
Research on inoculum and infection potential in soil and water on a field‐scale trails far behind that on airborne pathogens and is somewhat hampered still by techniques for detection and quantification • Excellent techniques such as Q‐PCR are now becoming more readily available although all approaches have weaknesses
• Current understanding of the dynamics of inoculum production and disease is still very poor and estimates of disease risks often very elementary (i.e. +/‐!)
Why is this important?
• Proper understanding of disease risks is the basis for disease management – currently there is the danger of being overly cautious, or worse still, not even trying to determine disease risks
• Again, accurate and timely diagnosis is vital
• A major weakness of many studies is the focus on single ‘pathogen’ species – next generation sequencing and new nested immunodiagnostics techniques may help address this
• Detection of ‘latent’ infection still a challenge
Detection, diagnostics and quantification
Mixed populations:
•
•
Oomycetes/Non‐oomycetes
Pathogens/Non‐pathogens
Inoculum:
•
•
•
•
Seasonal (seasonal susceptibility)
‘Spikes’
Density v. infection (thresholds?)
Latent (silent) infection (inc. seeds?)
Disease management & control
Recap on dispersal:
Management and control:
 Infested soil & media
 Fungicides & biocontrol agents
 Infested plants (& seeds)
 Sterilants/disinfectants
 Decaying infected material
 Avoidance, certification & HYGIENE
 Dust, debris , ‘dirt’
 Water management
 WATER
Avoidance and hygiene
Barriers to inoculum spread e.g. SpinOut‐treated matting
Quarantine
Clean equipment and containers etc.
Zero tolerance of puddles and debris
In HDC PC97 – reduced incidence of Pythium
infection by 70% Simple hygiene
Thank you !
Dr Tim Pettitt
University of Worcester
[email protected]